There is currently an increasing level of research activity in the area of alternative power sources for micro electrical mechanical systems (MEMS) devices, such devices being described in the art as being used for ‘energy harvesting’ and as ‘parasitic power sources’. Such power sources are currently being investigated for powering wireless sensors.
It is known to use an electromechanical generator for harvesting useful electrical power from ambient vibrations. A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core. The mass which is moved when vibrated is mounted on a cantilever beam. The beam can either be connected to the magnetic core, with the coil fixed relative to an enclosure for the device, or vice versa. The electromechanical generators are miniaturized. This makes them readily locatable in a variety of positions on or in a host apparatus for providing electrical power for driving single or plural components.
One such known miniature electromechanical generator is illustrated in FIG. 1. The known design for the electromechanical generator 2 has magnets 4, 6 attached to a flexible spring-steel beam 8 adjacent to a fixed copper coil 10 located between the magnets 4, 6. An opening 12 is formed in the beam 8 at a free end 14 thereof and the magnets 4, 6 are located on opposite sides of the opening 12. The coil 10 is disposed in the opening 12, and is mounted on an aim 16 extending upwardly from a base 18. The other end 20 of the beam 8 is fixed to an upright support 22 extending upwardly from the base 18. Each magnet 4, 6 comprises a pair of magnet elements 24, 26, each element 24, 26 being located on a respective upper or lower side of the beam 8, with the two elements 24, 26 of each pair being connected together by a keeper 28 located at a side remote from the coil 10. This creates a region of magnetic flux between the magnets 4, 6 in which the coil 10 is disposed.
When the electromechanical generator 2 is subjected to vibration in the vertical direction (see FIG. 1) and at a frequency near the resonance frequency of the assembly of the beam 8 and the magnets 4, 6, the beam 8 and magnets 4, 6 carried thereon oscillate relative to the coil 10. This movement results in a changing magnetic flux through the coil 10, and hence an induced voltage along the wire of the coil 10.
This known design is magnetically very efficient because of the lack of any significant conductive elements in the flux path, which would otherwise tend to support eddy currents. However, the low permeability (and hence high reluctance) path between the magnets 4, 6 leads to a low flow of flux and hence a low induced voltage per turn of the coil 10. To attempt to counteract the low induced voltage, the coil 10 is required to have many turns in a small volume so that the output voltage is at a sufficient value for a useful power output. This in turn results in a high coil resistance, which reduces the electrical efficiency of the electromechanical generator 2.
Also, the known electromechanical generator 2 requires a sprung beam 8, which acts as a cantilever beam, supporting the vibratable magnet assembly. Such a beam requires a suitable spring material to be provided and for the beam supporting the vibratable magnet assembly to be carefully tuned. This can be difficult to achieve accurately, and the resonance characteristics of the sprung beam can vary over the design lifetime of the electromechanical generator 2.
DE29618015U discloses an electrical generator for bicycles in which a magnet is mounted on a leaf spring that reacts to vibration and moves relative to a core to induce a voltage in a coil. This rudimentary disclosure does not relate to miniature generators as discussed hereinabove, or address or solve the problems discussed above with respect to the known electromechanical generator that requires a sprung beam which acts as a cantilever beam.
SU1075357A discloses a body oscillatory motion electric generator for charging a battery. A hinged body having an E-shaped magnetic circuit with a winding on the middle core and a permanent magnet on an outer core is supported for oscillatory motion by a helical spring. This disclosure does not address or solve the problems discussed above with respect to the known electromechanical generator that requires a sprung beam which acts as a cantilever beam.
SU776487A discloses an electrical generator for charging a cardio-simulator battery. The generator incorporates a rotatable cylindrical armature with a coil and conical magnet poles at its ends. This disclosure does not address or solve the problems discussed above with respect to the known electromechanical generator that requires a sprung beam which acts as a cantilever beam.
U.S. Pat. No. 5,180,939 discloses a mechanically commutated linear alternator incorporating a pair of reciprocating elements. This disclosure does not address or solve the problems discussed above with respect to the known electromechanical generator that requires a sprung beam which acts as a cantilever beam.
Accordingly, there is still a need to enhance the efficiency of the conversion by an electromechanical generator, in particular a miniature electromechanical generator, of mechanical vibration energy into electrical energy, and thereby into useful electrical power.
There is also a need for an electromechanical generator, in particular a miniature electromechanical generator, which overcomes or obviates the problems of sprung cantilever beams described above.